岩溶系统中土壤氮肥使用对岩溶碳汇的影响研究

In previous papers，it has been shown that the flux of the net atmospheric CO2 sink (i.e., karst carbon sink) caused by terrestrial carbonate weathering has a similar magnitude to that of the carbon sink in global forests. However, dissolution of the carbonate rocks by nitric acid as a result of the overuse of N-fertilizers in agricultural areas will greatly weakened karst carbon sinks. Using a small amount of N-fertilizers can increase microbial activities and enhance soil carbon cycle, thus promote karst process in the soil, but its process and mechanism and its effect on karst carbon sink need to be clarified immediately. Through comparison experiment and hydrogeochemistry monitoring in a karst catchment with agricultural activities, this research will study the effects of N-fertilizers of different concentration on crop production, soil CO2 which can accelerate karst process and soil respiration which release CO2 to atmosphere. At the same time, its effects on the rate of soil nitrification, buffering capacity of karst soil with high base saturation to H+ induced by soil nitrification will be revealed, so as to calculate the rate between [H+] in change with base cation and [H+] used by carbonate dissolution. Then we can estimate the amount of decreased karst carbon sink flux owing to carbonate dissolution of H+ produced by nitrification. At last, we will comprehensively evaluate the effects of using N-fertilizers on catchment karst carbon sink and search for a balance point between increasing crop production and keeping more karst carbon sink flux, which can provide significant guidance for increasing the accuracy of estimating the amount of atmospheric CO2 sink caused by carbonate weathering and for how to use N-fertilizers correctly.

有资料显示陆地碳酸盐风化形成的大气CO2净汇（岩溶碳汇）与世界森林碳汇量级相当。但农业地区过量氮肥使用形成的硝酸对碳酸盐溶解会大大减弱岩溶碳汇，而少量N肥可促进土壤微生物活性，增强土壤碳循环过程，促进土下岩溶作用的发生，但其过程与机理及对岩溶碳汇的影响急需澄清。本研究拟以具有农业活动的岩溶流域为例，通过对比实验及流域水文地球化学观测，一方面研究不同施氮水平对作物增产、土壤CO2浓度（促进岩溶作用，增加碳汇）及土壤呼吸（释放CO2，减少碳汇）的影响；另一方面研究其对土壤硝化速率及岩溶土壤高的盐基饱和度对硝化产H+缓冲能力的影响，计算被盐基离子交换H+与被淋滤H+的比例及硝化产H+对碳酸盐溶解的减汇量。最后综合评价氮肥使用对流域岩溶碳汇的效应，寻找氮肥使用对作物增产与调节岩溶碳汇的平衡点，为提高岩石风化碳汇评价结果的精度及合理使用氮肥进行碳汇的人为调控提供重要的指导意义。

农业地区过量氮肥使用形成的硝酸对碳酸盐溶解会减弱7%-17%的岩溶碳汇量。适量氮肥的施用，能提高土壤CO2浓度，从而提高土下碳酸盐岩的溶解速率。因此，要从两方面分析岩溶系统中土壤氮肥施用对岩溶碳汇效应的影响。同时，岩溶区碳酸盐岩风化形成的土壤具有较高的pH值及盐基饱和度，对H+有较强的缓冲作用，运用端元法可能过高估算了硝酸对碳酸盐岩的溶解量。本研究以具有农业活动的岩溶流域为例，通过对比实验及流域水文地球化学观测，得到以下主要结果： .岩溶土壤盐基离子对硝化产酸具有缓冲能力：在土壤pH为5.5-7时，发生盐基离子直接缓冲外源质子。在pH为4.36-5.5时，盐基离子直接缓冲和土壤溶液中溶解的氧化铝中和同时存在，但仍以盐基离子缓冲为主。施肥增加了岩溶碳循环强度：土壤CO2浓度、土壤碳排放及试片溶蚀速率最高分别提高了103.77%，136.64%和28.31%。. 土壤硝化作用产生的H+在剖面上被盐基离子全部缓冲，不能到达基岩层对碳酸盐岩产生溶蚀。灰色关联度分析法表明22 g N/m2为减少碳排放，增加岩溶作用的最适施肥浓度。. 碳同位素计算结果表明碳酸盐岩碳库对地下水δ13CDIC的贡献比为43.37%，土壤CO2碳库的贡献比为56.63%，而水化学法的结果显示碳酸盐岩碳库对地下水DIC的贡献比为56.88%，其中碳酸溶蚀碳酸盐岩贡献为86.24%，硝酸溶蚀贡献为13.76%。土壤/大气CO2碳库对地下水DIC的贡献比为43.12%。土壤中流失的[Ca2++Mg2+]的量是地下水中△[Ca2++Mg2+]的21.6倍，说明地下水中[Ca2++Mg2+]的增加是土壤淋滤交换出的钙镁的结果，而不是硝酸溶蚀的结果。因此，对于本流域，传统的水化学法高估了13.76%硝酸溶蚀贡献，低估了13.51%土壤CO2碳库对地下水DIC的贡献。结果为提高岩石风化碳汇评价结果的精度及合理使用氮肥进行碳汇的人为调控提供了重要线索。